ac·rit·arch (ak´ri tärk´) n. in palæontology, a microscopic portions of various classes of organism, which can be encountered in macerated sedimentary rock when searching for spores or pollen. b any of a proposed grouping of ancient marine eukaryotes, perhaps associated with the origin of dinoflagellates. [< NL Acritarcha Evitt < Gk. ákrit(os) άκριτοςunarranged, undistinguishable + archḗ αρχήbeginning] —ac´ri·tar´chous,adj.

ac·tin (ak´tin) n. a protein which is capable of providing motion to liquids and organelles within a living cell. [< Gk. aktinos ακτῑνοςa ray, beam + NL -in chemical suffix for an activator]

ad·der·wort (ad´ər wôrt´) n. bistort; snakeweed.

adze (adz) n. a tool consisting of a steel blade presented crossways to an oblong wooden handle, which can be used to shape a felled tree – or large piece of wood – by creating a flattened surface. Adzes are frequently employed in boat-building. [< OE adesa]

ær·a·tor (ãr´ā tər) n.1 a metal implement which punctures the sod to allow access of air. 2 a device which introduces turbulence into flowing water to allow access of air. 3 a device which stirs grain stored in a granary, to allow access of air and prevent damage by fungus and insect.

aff.Abbrev. akin to, often used prior to an existing Linnaean binomial, in the case where a provisional taxon appears to have no correct existing name, but resembles the species indicated. [< L affinis neighbouring, related, akin]

a·gar·wood (ä´gär wu̇d´) n. the dark resinous heartwood of certain species of the genera Aquilaria Lam., Gyrinops Gaertn., and especially Lachnolepis moluccana Miq. (all of the Thymelaeaceae), which forms in response to infection by an ascomycete; oud. It is highly valued as a fragrance.

air layerv.t.air layered, air layering. the inducement of new roots in a shrub or tree, by cutting an incision in the bark above ground and packing the exposed tissue with soil or peat to encourage development of roots. The altered branch can then be removed and planted elsewhere.

aj·o·wan (ä´djō wän´) n.1 ammi. 2 any of various related species of the genera Trachyspermum Link, Sison L., and Carum L. (all of the Apiaceae), and native to ranges in Africa, the middle east, and central Asia; carom. All produce fruits bearing oils rich in thymol, and are of use in medicine and as spice. [< Hindi ajvāyn]

alley croppingn. a method of establishing sustainable agriculture in an area of tropical forest which has previously sustained slash and burn exploitation. It consists of planting alleys of beneficial tree species (often, species of Inga Mill., of the Fabaceae: Mimosoideae) in rows at approximately 4m intervals, and growing crops in the soil between the rows.

al·loch·tho·nous (ə läk´thə nəs) adj. in the interpretation of fossils, specimens found in a place other than where they were naturally found living and growing. [< Gk. allokhthon ἄλλοχθών< allos ἄλλοςanother, different, strange + khthonos χθονόςearth, soil]

all·spice (äl´spīs) n.1 an aromatic berry whose ground seeds, or the fruit in its entirety, are the source of a spice. 2 the tree upon which these fruits are borne (Pimenta dioica (L.) Merr., of the Myrtaceae), native principally to Jamaica. One variety (P. dioica (L.) Merr. var. tabasco (Schltdl. & Cham.) Standl.) is often used in Tabasco. [named for its apparent combination of flavours from cinnamon, nutmeg, and cloves]

am·la (äm´lä) n. a small tree native to the east Indies (Phyllanthus emblica L., of the Euphorbiaceae), bearing edible berries which are sour and astringent. The plant has been used for a wide variety of medicinal purposes, with particular effectiveness against pancreatitis. [< Sanskrit amalaki]

am·mi (ä´mē) n. an African species (Sison ammi L., of the Apiaceae), bearing fruits whose oil is rich in thymol, and is of use in medicine and as a spice; ajowan; carom. [Gk. ἄμμι]

am·phi·tro·pous (am´fē trō´pəs or am´fit´ rə pəs) adj. of an ovule, being partially inverted shortly after its development begins so that its micropyle is situated to one side funiculus attaches to the carpel or ovary. The radicle of the ovule remains near the attachment of the funiculus to the ovule, and it is the funiculus which is bent through approximately 90°. [< Gk. amphi ἀμφίaround + tropos τρόποςturn, change in manner] —am´phi·tro´pic,adj.

a·na·tro·pous (an´ə trō´pəs or ə nat´ rə pəs) adj. of an ovule, being completely inverted shortly after its development begins so that its micropyle is situated near the place where the funiculus attaches to the carpel or ovary. The radicle of the ovule remains near the attachment of the funiculus to the ovule, and it is the funiculus which is bent through 180°. [< Gk. ana ἄναwithout + tropos τρόπος turn, change in manner] —a´na·tro´pic,adj.

anchovy pearn. the fruit of a tree native to the West Indies (Grias cauliflora L., of the Lecythidaceae), which bears fleshy capsules containing several large woody seeds, often used as a source of oil; river pear.

an·ge·li·que (än´dje lē´ke) n.1 a hard reddish wood very suitable for shipbuilding due to its durability. 2 the trees which produce this wood (Dicorynia paraensis Benth., as well as the closely-related D. guianensis Amshoff, both of the Fabaceae: Caesalpinioideae), native to Brazil and the Guianas. 3Rare. angelica. [< F angelique angelica]

an·gi·o·car·pous (an´djē ō kär´pəs) adj.1 of a fruit, being borne partially or wholly enclosed by a receptacle or husk. 2 of a fungus or lichen, having the reproductive structures embedded in or enclosed in the thallus. [< Gk. angeion αγγεῖονvessel + karpos καρπος fruit + -ōsus -ωσιςprone to] —an´gi·o·car´pic,adj.

an·ti·cli·nal (an´tē klī´nəl) adj.1 of the plane of cellular division, oriented perpendicular to the surface of an organ (such as the meristem), so that additional cells are produced laterally. 2 of a cell wall, oriented perpendicular to the surface of an organ as a result of such growth. [< Gk. antí ἀντίagainst, opposite of + klinein κλίνεινlean] —an´ti·cli´nal·ly,adv.

ap·o·pet·al·ous (ap´ō pet´əl əs) adj. of a taxon or its flowers, having the petals tending to be reduced in number as an evolutionary trend; secondary apetaly. [< Gk. apo-ἀπό-from, away from + petalon πέταλον flower leaf + -ōsus-ωσιςprone to] —ap´o·pet´al·y,n.

ap·pres·so·ri·um (a´pre sōr´ē üm or -ē əm) n.-ri·a. a structure formed at the tip of infectious or mycorrhizal mycelia which come into contact with a host plant surface. It consists of a flattened cytoplasmic pad which settles upon the tissue surface (attracted by chemical cues), and a penetration peg which penetrates the cuticle and cell walls of the host plant. [NL < L appressus press upon, clench + -or agent + NL -ium locative suffix]

ar·bor·ist (är´bə rist´) n. one who is a specialist in the growth and care of trees, including treatment of nutritional deficits and infestations. [< F arboriste]

ar·bus·cle (är bus´kəl) n.1 a dwarf tree, one between a shrub and tree in size, as a penjing. 2 a growth form of endomycorrhizæ, where – once a mycelium enters a plant cell – it forms a short-lived branching outgrowth within that cell. [< L arbuscula small tree] —ar·bus´cu·lar,adj.

ar·row·grass (ãr´ō gras´) n.1 any member of the genus Scheuchzeria L. (of the Scheuchzeriaceae), which grows in peat bogs of the northern hemisphere. It is a low herb whose leaves are linear and sheathing, and whose flowers bear 2 whorls of tepals, 2 whorls of stamens, and 2 whorls of 3 free carpels. 2 any of a number of species of the genus Triglochin L. (of the Juncaginaceae), upright graminoid semiaquatic perennial herbs frequent in marshes worldwide. [descriptive of the open capsule segments of T. palustris L.]

as·pect (as´pekt) n.1 the appearance or quality of an individual organism or community, at a particular time. 2 the direction towards which a sloping substrate faces. 3 the specific way in which an object or mental construction is considered. [ME < L aspectus appearance, visible form] —as·pec´tu·al,adj.

auct. (auc tō´rüm) Abbrev.n. author, or authors, used in Linnaean nomenclature for an unspecified author or authors subsequent to the recognized authority, who has or have misapplied the preceding name. Occasionally, such an abbreviation used in place of the recognized authority will be followed by Latin non (not) and the recognized authority. [L auctorum of author(s)]

auct.mult. (auc tō´rüm mül tō´rüm) Abbrev.n. multiple authors, used in place of an authority in Linnaean nomenclature, specifically in the case where the preceding name has been previously misapplied in several distinct contexts by several authors. [L auctorum of author(s) + multorum many]

au·to·ly·sin (o´tə lī´sin or o´täl´i sin) n. any of a chemical group of enzymes which are capable of destroying the cell walls or tissues of an organism within which they are produced. [< G autolysis + NL -ina noun suffix denoting organic substances or compounds]

aux·in (ok´sin) n. a plant hormone which promotes elongation of cells in principle axes such as roots and shoots, and also augments growth events in buds as well as playing a rôle in dehiscence. [< G < Gk. auxē αὔξηgrowth, increase + NL -ina noun suffix denoting organic substances or compounds] —aux´in·ic,adj.

aux·o·troph (ok´sə trōf´) n. a mutant microorganism which has distinct nutritional requirements from those of the parent strain. [< Gk. auxē αὔξηgrowth, increase + trophos τροφός one who feeds] —aux´o·tro´phic,adj.

axe (aks) n. a tool consisting of a steel blade parallel to – but presented at right angles to – an oblong wooden handle, which can be used to cut down a tree by chopping through its trunk. [OE æx < Gmc.]